1. Field of the Invention
The subject invention is directed to inertial sensors, and more particularly, micro-electric mechanical sensors (MEMS) used for inertial guidance systems, and to a method of using inertial information stored in a proof mass of a closed-loop inertial sensor in saturation and observed as proof mass displacement, wherein the closed-loop response is added with an open-loop response to extend the range of the sensor.
2. Description of Related Art
Microelectromechanical systems (MEMS) sensors are often used in inertial guidance systems to sense the acceleration or orientation of a device, such as a spacecraft, aircraft or land based vehicle, along one, two, or three axes or directions. These sensors are typically constructed with a proof mass mounted to a substrate by way of a compliant suspension. The proof mass is sensitive to inertial forces, such that movement of the device results in displacement of the proof mass. The displacement is proportional to the amount of acceleration the device is experiencing, and is typically measured by capacitive sensor plates disposed beneath the proof mass.
In designing these sensors, the stiffness of the suspension is determined by the range of inertial force that will displace the proof mass and the electrostatic force from the capacitive sensor plates that will pull the proof mass with increasing force as displacement occurs. Ultimately, the measurement range of an inertial sensor operating in an open-loop configuration is limited so as to ensure that a “snap-down” of the proof mass will not occur. In addition, there is a trade-off between measurement sensitivity and desired range. Open loop operation provides the simplest electronics solution, with only displacement sensing required. However, the electrostatic forces produce large non-linear output.
MEMS devices that require improved linearity are typically used in a closed-loop configuration. In this implementation, the proof mass displacement is sensed as in an open-loop application, but a feedback electrostatic force is also applied to the proof mass to null out the proof mass displacement. The output of the sensor is the voltage applied to the electrodes to null the proof mass displacement. The result is improved linearity since the proof mass is only exercised with small displacements. The range of the sensor is limited to the electrode characteristic (i.e., area, gap, etc.) and the voltage available to be applied as feedback force.
While closed-loop operation has its performance advantages, under some conditions, a much higher sensing range is desired than the range that can be provided using a closed-loop configuration. The subject invention provides a system and method for providing such high range capability with a closed-loop inertial sensor, by combining the closed-loop output response of the sensor with an open-loop displacement output.